FIG. 1 shows an exemplary network structure of an Evolved Universal Mobile Telecommunications System (E-UMTS) as a mobile communication system to which a related art and the present invention are applied. The E-UMTS system is a system that has evolved from the existing UMTS system, and its standardization work is currently being performed by the 3GPP standards organization. The E-UMTS system can also be referred to as a LTE (Long-Term Evolution) system.
The E-UMTS network can roughly be divided into an E-UTRAN and a Core Network (CN). The E-UTRAN generally comprises a terminal (i.e., User Equipment (UE)), a base station (i.e., eNode B), an Access Gateway (AG) that is located at an end of the E-UMTS network and connects with one or more external networks. The AG may be divided into a part for processing user traffic and a part for handling control traffic. Here, an AG for processing new user traffic and an AG for processing control traffic can be communicated with each other by using a new interface. One eNode B may have one or more cells. An interface for transmitting the user traffic or the control traffic may be used among the eNode Bs. The CN may comprise an AG, nodes for user registration of other UEs, and the like. An interface may be used to distinguish the E-UTRAN and the CN from each other.
The various layers of the radio interface protocol between the mobile terminal and the network may be divided into a layer 1 (L1), a layer 2 (L2) and a layer 3 (L3), based upon the lower three layers of the Open System Interconnection (OSI) standard model that is well-known in the field of communications systems. Among these layers, Layer 1 (L1), namely, the physical layer, provides an information transfer service to an upper layer by using a physical channel, while a Radio Resource Control (RRC) layer located in the lowermost portion of the Layer 3 (L3) performs the function of controlling radio resources between the terminal and the network. To do so, the RRC layer exchanges RRC messages between the terminal and the network. The RRC layer may be located by being distributed in network nodes such as the eNode B, the AG, and the like, or may be located only in the eNode B or the AG.
FIG. 2 shows exemplary control plane architecture of a radio interface protocol between a terminal and a UTRAN (UMTS Terrestrial Radio Access Network) according to the 3GPP radio access network standard. The radio interface protocol as shown in FIG. 2 is horizontally comprised of a physical layer, a data link layer, and a network layer, and vertically comprised of a user plane for transmitting user data and a control plane for transferring control signaling. The protocol layer in FIG. 2 may be divided into L1 (Layer 1), L2 (Layer 2), and L3 (Layer 3) based upon the lower three layers of the Open System Interconnection (OSI) standards model that is widely known in the field of communication systems.
Hereinafter, particular layers of the radio protocol control plane of FIG. 2 and of the radio protocol user plane of FIG. 3 will be described below.
The physical layer (Layer 1) uses a physical channel to provide an information transfer service to a higher layer. The physical layer is connected with a medium access control (MAC) layer located thereabove via a transport channel, and data is transferred between the physical layer and the MAC layer via the transport channel. Also, between respectively different physical layers, namely, between the respective physical layers of the transmitting side (transmitter) and the receiving side (receiver), data is transferred via a physical channel.
The Medium Access Control (MAC) layer of Layer 2 provides services to a radio link control (RLC) layer (which is a higher layer) via a logical channel. The RLC layer of Layer 2 supports the transmission of data with reliability. It should be noted that if the RLC functions are implemented in and performed by the MAC layer, the RLC layer itself may not need to exist. The PDCP layer of Layer 2 performs a header compression function that reduces unnecessary control information such that data being transmitted by employing Internet Protocol (IP) packets, such as IPv4 or IPv6, can be efficiently sent over a radio interface that has a relatively small bandwidth.
The Radio Resource Control (RRC) layer located at the lowermost portion of Layer 3 is only defined in the control plane, and handles the control of logical channels, transport channels, and physical channels with respect to the configuration, re-configuration and release of radio bearers (RB). Here, the RB refers to a service that is provided by Layer 2 for data transfer between the mobile terminal and the UTRAN.
As for channels used in downlink transmission for transmitting data from the network to the mobile terminal, there is a Broadcast Channel (BCH) used for transmitting system information, and a downlink Shared Channel (SCH) used for transmitting user traffic or control messages. A downlink multicast, traffic of broadcast service or control messages may be transmitted via the downlink SCH or via a separate downlink Multicast Channel (MCH). As for channels used in uplink transmission for transmitting data from the mobile terminal to the network, there is a Random Access Channel (RACH) used for transmitting an initial control message, and an uplink Shared Channel (SCH) used for transmitting user traffic or control messages.
As for downlink physical channels for transmitting information transferred via the channels used in downlink transmission over a radio interface between the network and the terminal, there is a Physical Broadcast Channel (PBCH) for transmitting BCH information, a Physical Multicast Channel (PMCH) for transmitting MCH information, a Physical Downlink Shared Channel (PDSCH) for transmitting PCH and a downlink SCH information, and a Physical Downlink Control Channel (PDCCH) (also, referred to as ‘DL L1/L2 control channel’) for transmitting control information provided by the first and second layers such as a DL/UL Scheduling Grant, and the like. As for uplink physical channels for transmitting information transferred via the channels used in uplink transmission over a radio interface between the network and the terminal, there is a Physical Uplink Shared Channel (PUSCH) for transmitting uplink SCH information, a Physical Random Access Channel (PRACH) for transmitting RACH information, and a Physical Uplink Control Channel (PUCCH) for transmitting control information provided by the first and second layers, such as a HARQ ACK or NACK, a Scheduling Request (SR), a Channel Quality Indicator (CQI) report, and the like.
In LTE system, a HARQ operation is performed in a MAC (Medium Access Control) layer for an effective data transmission. The following is a detailed description of the HARQ operation.
FIG. 4 is an exemplary view showing a HARQ operation method for an effective data transmission. As illustrated in FIG. 4, a base station (or eNB) may transmit downlink scheduling information (referred as ‘DL scheduling information’ hereafter) through a PDCCH (Physical Downlink Control Channel) in order to provide data to a terminal (UE) during a HARQ operation. The DL scheduling information may include a UE identifier (UE ID), a UE group identifier (Group ID), an allocated radio resource assignment, a duration of the allocated radio resource assignment, a transmission parameter (e.g., Modulation method, payload size, MIMO related information, etc), HARQ process information, a redundancy version, or a new data indicator (NID), etc. Usually, the terminal (UE) performs multiple HARQ processes, the multiple HARQ processes are operated synchronously. Namely, each HARQ process is allocated synchronously in every transmission time interval (TTI). For example, a HARQ process 1 may perform in a first transmission time interval (TTI 1), a HARQ process 2 may perform in TTI 2, . . . , a HARQ process 8 may perform in TTI 8, the HARQ process 1 may again perform in TTI 9, and the HARQ process 2 may again perform in TTI 10, etc. Since the HARQ processes are allocated in synchronous manner, a certain HARQ process associated with a TTI which receives a PDCCH for initial transmission of a particular data may be used for such data transmission. For example, if the terminal receives a PDCCH including an uplink scheduling information in Nth TTI, the terminal may actually transmit a data in N+4 TTI.
The HARQ retransmission of the terminal is operated in a non-adaptive manner. That is, an initial transmission of a particular data is possible only when the terminal receives a PDCCH including an uplink scheduling information. However, the HARQ retransmission of the data can be possibly operated without receiving the PDCCH, as next TTI allocated to a corresponding HARQ process can be used with same uplink scheduling information. Here, transmission parameters may be transmitted through a control channel such as a PDCCH, and these parameters may be varied with a channel conditions or circumstances. For example, if a current channel condition is better than a channel condition of an initial transmission, higher bit rate may be used by manipulating a modulation scheme or a payload size. In contrast, if a current channel condition is worst than a channel condition of an initial transmission, lower bit rate may be used.
The terminal checks an uplink scheduling information by monitoring a PDCCH in every TTI. Then, the terminal transmits data through a PUSCH based on the uplink scheduling information. The terminal firstly generates the data in a MAC PDU format, and then stores it in a HARQ buffer. After that, the terminal transmits the data based on the uplink scheduling information. Later, the terminal waits to receive a HARQ feedback from a base station (eNB). If the terminal receives a HARQ NACK from the base station in response to the transmitted data, the terminal retransmits the data in a retransmission TTI of a corresponding HARQ process. If the terminal receives a HARQ ACK from the base station in response to the transmitted data, the terminal terminates to operate the retransmission of the HARQ. The terminal counts a number of transmissions (i.e. CURRENT_TX_NB) whenever the data is transmitted in a HARQ process. If the number of transmissions is reached to a maximum number of transmissions, which set by an upper layer, data in the HARQ buffer is flushed.
The HARQ retransmission is performed according to a HARQ feedback from a base station, a data existence in the HARQ buffer, or a transmission time of a corresponding HARQ process. Here, each of HARQ process may have a HARQ buffer respectively. The value in the NDI (New Data Indicator) field contained in the PDCCH may be used for the UE to determine whether the received data is an initial transmission data or a retransmitted data. More specifically, the NDI field is 1 bit field that toggles every time a new data is transmitted or received. (0→1→0→1→ . . . ) As such, the value in the NDI for the retransmitted data always has a same value used in an initial transmission. From this, the UE may know an existence of retransmitted data by comparing these values.
Description of an uplink timing alignment maintenance in a LTE system will be given. In the LTE system that based on an Orthogonal Frequency Division Multiplex (OFDM) technology, there is possibility of interferences between terminals (UEs) during a communication between UE and base station (eNB). In order to minimize interferences between terminals, it is important that the base station must manage or handle a transmission timing of the UE. More particularly, the terminal may exist in random area within a cell, and this implies that a data transmission time (i.e., traveling time of data from UE to base station) can be varied based on a location of the terminal. Namely, if the terminal is camped on edge of the cell, data transmission time of this specific terminal will be much longer than data transmission time of those terminals who camped on a center of the cell. In contrast, if the terminal is camped on the center of the cell, data transmission time of this specific terminal will be much shorter than data transmission time of those terminals who camped on the edge of the cell. The base station (eNB) must manage or handle all data or signals, which are transmitted by the terminals within the cell, in order to prevent the interferences between the terminals. Namely, the base station must adjust or manage a transmission timing of the terminals upon each terminal's condition, and such adjustment can be called as the timing alignment maintenance. One of the methods for maintaining the timing alignment is a random access procedure. Namely, during the random access procedure, the base station receives a random access preamble transmitted from the terminal, and the base station can calculate a time alignment (Sync) value using the received random access preamble, where the time alignment value is to adjust (i.e., faster or slower) a data transmission timing of the terminal. The calculated time alignment value can be notified to the terminal by a random access response, and the terminal can update the data transmission timing based on the calculated time alignment value. In other method, the base station may receive a sounding reference symbol (SRS) transmitted from the terminal periodically or randomly, the base station may calculate the time alignment (Sync) value based on the SRS, and the terminal may update the data transmission timing according to the calculated time alignment value.
As explained above, the base station (eNB) may measure a transmission timing of the terminal though a random access preamble or SRS, and may notify an adjustable timing value to the terminal. Here, the time alignment (Sync) value (i.e., the adjustable timing value) can be called as a time advance command (referred as ‘TAC’ hereafter). The TAC may be process in a MAC (Medium Access control) layer. Since the terminal does not camps on a fixed location, the transmission timing is frequently changed based on a terminal's moving location and/or a terminal's moving velocity. Concerning with this, if the terminal receives the time advance command (TAC) from the base station, the terminal expect that the time advance command is only valid for certain time duration. A time alignment timer (TAT) is used for indicating or representing the certain time duration. As such, the time alignment timer (TAT) is started when the terminal receives the TAC (time advance command) from the base station. The TAT value is transmitted to the terminal (UE) through a RRC (Radio Resource Control) signal such as system information (SI) or a radio bearer reconfiguration. Also, if the terminal receives a new TAC from the base station during an operation of the TAT, the TAT is restarted. Further, the terminal does not transmit any other uplink data or control signal (e.g., data on physical uplink shared channel (PUSCH), control signal on Physical uplink control channel (PUCCH)) except for the random access preamble when the TAT is expired or not running.
In general, a MAC layer of the terminal and base station handles a time alignment (synchronize) management. Namely, The TAC is generated in the MAC layer of the base station, and the MAC layer of the terminal receives the TAC through a MAC message from the base station. However, because the TAC is received by the MAC message, a transmission of the TAC is not fully guaranteed. For example, the base station transmits the MAC message including the TAC in a HARQ process, and the terminal attempts to receive the data. The terminal transmits a NACK signal to the base station if the terminal fails to decode the data. However, if such NACK signal is mistakenly treated as an ACK signal by the base station, a TAT of the base station is restarted whereas a TAT of the terminal is not restarted. Thusly, a failed synchronization can be happened between the terminal and base station.
Another example of drawback in a related art can be given as following. Firstly, the terminal receives an uplink scheduling information through a PDCCH for a transmission of data 1. Then, the terminal transmits the data 1 to the base station using the HARQ process. In response to the transmitted data 1, the terminal receives a NACK from the base station. Therefore the terminal has to retransmit the data 1, however, the TAT of the terminal can be expired before a retransmission of the data 1. In this situation, the terminal can not possibly retransmit the data 1 due to expiry of the TAT. Therefore, the terminal restarts the TAT after receiving a TAC from the base station though a random access channel (RACH) procedure. However, the terminal still transmits data 1 at a transmission timing of the HARQ process because the data 1 is still stored in a HARQ buffer of the terminal. In this case, the transmission of the data 1 is not expected by the base station, this data transmission can be collided with other data transmission by other terminals.